ReferenceData's structure is very similar to ClinicalData's structure, except that ReferenceData has no “SubjectData”, no “StudyEventData”, and no “FormData”. It only has “ItemGroupData and ItemData”.
So essentially it is transporting 2-dimensional data, i.e. tables.
And I think that is also its primary usage: transport of tables between applications between about data for which there are no subjects, no visits and no forms.

Of course a reference data point only usually makes sense when it has units attached, otherwise we do not know what we exactly are talking about.

So, what does this say?
Not much, except that it represents SOME table data. There is little information in it, as the OIDs are arbitrary, so we cannot rely them to have a meaning. For that we need metadata.
Now the specification also states:

”Since reference data can be independent of any particular study, it may be desirable to keep the reference metadata separate from clinical metadata. This can be done by creating a Study element with no Protocol, StudyEventDef, or FormDef elements. All the ItemGroupDefs would have IsReferenceData=Yes. Such a study would have no clinical data.”.

OK, let's do so. We create a study with OID “ReferenceDataStudy”, with a MetaDataVersion “MV.REFDATA” which ONLY contains ItemGroupDef elements and ItemDef elements, and maybe “CodeList” elements. For our example, such a file could look like (ODM element omitted here):

If we also want to transport the reference data together with a real study, we can easily do so for as well the definitions, as for the reference data themselves, the definitions being incorporated through the ”Include” element: